DESCRIPTION: Infection of neurons by varicella zoster virus (VZV) is not well understood, although it is a serious medical problem. VZV infects and becomes latent in dorsal root ganglia (DRG) during varicella. Subsequent reactivation later in life causes zoster. Zoster has been difficult to study because of the limited host range of VZV and the consequent lack of suitable animal or in vitro neuronal models. Identification of viral genes expressed during latency has been complicated by the problem of distinguishing latent from reactivating virus in autopsied ganglia. They propose to take advantage of an in situ hybridization-based method that permits human DRG neurons harboring latent or reactivating VZV to be distinguished. To identify viral genes which may, when expressed, maintain latency, they will analyze gene expression in individual human DRG cells which are determined in serial section to contain latent or reactivating VZV. A multiplex method of in situ hybridization will be employed to identify every VZV open reading frame (ORF) expressed during latent infection. Transcription activity of these ORFs will be quantified by RT-PCR. Advantage will also be taken of the recent demonstration that VZV establishes latent infection in rat DRG. To validate the use of rat DRG as a model of VZV latency, they will compare VZV genes expressed during latency in rat neurons with those expressed in their human counterparts. The relative contributions made by viremia and retrograde transport in enabling VZV to infect rat DRG cells will be studied. Finally, the relative abilities of wild-type VZV (WT) and vaccine-type VZV (VT) to establish latent infection in rat DRG and to reactivate will be determined. The ability of VT to establish latency is important because of the possibility that latent infection contributes to long-term protection from VZV reinfection. VZV latency will also be examined in hNT neurons (postmitotic human neurons that differentiate in vitro from proliferating neural precursors). They will confirm preliminary data that VZV establishes latent infection in HNT neurons and then investigate gene expression during latency, and the comparative virulence of WT and VT.